Both Primal and Aether are in the same data center. However, the server clusters have different login servers. Aether is getting hammered, while Primal is getting hammered slightly less and is taking the beating a lot better.

I think I remember that each server can handle 7500 or so concurrent logins before it starts queuing people. The queues for Balmung and Gilgamesh are so long that it's causing the login server to buckle under the stress, which in turn is screwing all the other lower population servers over, too.